Certain liquids, particularly radio-active waste water from nuclear installations, may be purified by means of a two-stage expansion-type evaporation installation, In such an arrangement radio-active liquid is circulated by a first circulating pump through a heat exchanger, in which evaporation heat is added, to the first stage in which a part of the liquid evaporates, the remaining liquid is recirculated by means of the first pump to the heat exchanger. A mixing condenser disposed intermediate the first and second stages receives the vapor exiting from the first stage, and such vapor is condensed therein by contact with a portion of the liquid present in the second stage, such portion being cycled into the mixing condenser by means of a second circulating pump. The so-condensed fluid in the mixing condenser is then introduced into the second stage itself.
The second pump maintains a counter-pressure in the mixing condenser which is normally equal to the pressure difference between the first and second stage; during normal operation, such first stage is normally maintained at a higher pressure than the second stage by the higher temperature of the vapor of the first stage.
Unfortunately, in case of failure of the second pump for any reason, the counter-pressure in the mixing condenser produced by this pump disappears and, thereby, the pressure in the first stage is suddenly lowered to that of the second stage (typically 1 atmosphere). Since the temperature existing in the first stage continues to correspond to the original pressure, the evaporation rate in the first stage suddenly increases at the time of such pressure drop, and the resulting high flow rate carries insufficiently purified fluid from the first stage to the second stage contaminating the second stage. The necessity of additional purification is costly and expensive leading to an undesired shutdown of the entire evaporation installation.